1. Introduction
Urban sprawl, broadly defined as dispersed, excessive, and wasteful urban growth, characterized by the excessive use of land for the building of single-family houses in the suburbs [
1], is increasingly observed in recent years in African cities. This urban explosion has placed the problem of the surge of populations on the urban outskirts at the center of debates on the city. While the developed countries are collapsing in the face of the crisis in the suburbs marked by violence and bad living, the population of the countries of the South exerts strong pressure on the outskirts of cities [
2], since the actual infrastructure built without any official urban plan do not provide for any urban service let alone the preservation of the surrounding ecology [
3]. The main cause of these pressures is the rapid increase in local populations [
4] combined with the absence of city extension policies or the implementation of certain master plans [
5].
Africa’s high population growth rate makes the demographic explosion one of the most important causes of land use changes in African cities. The latter is characterized by a peri-urban ring, which is a transition between the rural environment and the urban environment [
6,
7,
8]. Under the weight of vertiginous demographic growth, the peri-urban zone is receding, giving way to an urbanized area and, in turn, transforming its periphery, which was once rural. The urbanization of peripheral zones appears to be the essential form of growth for West African cities. It is manifested everywhere by a sprawl of residential areas, which sometimes pushes the limits of the city to considerable distances from the urban center [
9]. Despite multifaceted consequences, such as the housing and transport crisis, precarious employment, and a lack of sanitation [
10], climatic factors in general and heat islands in particular [
11] are considered today as consequences of a reduction in vegetation cover in residential areas.
Many studies on the dynamics of land use and land cover in urban areas have been performed during the last decades in various parts of the world. For instance, in Latin America, ref. [
12] analyzed the land cover dynamics along the urban–rural gradient of the Port-au-Prince agglomeration (Republic of Haiti) from 1986 to 2021 and found that the landscape has undergone significant changes because of the “high demand for housing” while in Asia, ref. [
13] estimated, determined the patterns, and identified the potential drivers of land-use changes during 1995–2015 in an urbanizing tropical watershed in Indonesia. They found, among other results, a major change from agricultural to urban areas in the study area. In Europe, the relationships between the spatial and temporal dynamics of land use and land cover (LULC), the hydro-geomorphological processes, and their impacts were evaluated by [
14]. They showed a highlighted increase in artificial areas for the period 1958–2018. In West Africa, land use and land cover dynamics were analyzed in Calabar Metropolis (Nigeria) by [
15] using a combined approach of remote sensing and a geographic information system. Their studies showed an increased trend in built-up areas from 2002 to 2016. Moreover, by analyzing the global satellite data of 120 cities, ref. [
16] found that cities “fragmented” a large area of landscapes. With the urban extension that can be observed everywhere in the world, the monitoring of territorial dynamics has taken an important place in the context of urban planning. It then appeared necessary to have reliable, precise, and continuously updated data on the evolution of the territory [
17,
18].
However, little has been done to understand land use and land cover dynamics in the urban areas of Greater Lomé. Thus, the main objective of this work is to understand the spatial and temporal dynamics of built-up areas in Greater Lomé with a view to providing guidelines for sustainable urban planning in the study area. Specifically, it aims to analyze (i) land use and land cover changes in relation to the evolution of buildings as well as (ii) trends in the annual rate and change matrix of LULC.
The city of Lomé is today the largest city in Togo whose development exceeds all forecasts. The observation is the strong spatial growth due to demographic pressure and the need for city dwellers to find housing. The majority of housing is built through the informal sector in the city center or on its outskirts, which gives rise to spontaneous outlying districts. The urban policies of Togo are those where the public power of urbanization is out of phase with the occupation of spaces by the population. In Lomé, the development of outlying districts was the work of customary landowners outside of any control by the state and local authorities. It was favored by the housing problems, which continue to worsen. These owners did not comply with the subdivision procedures provided for by law, in particular, obtaining the agreement of the minister in charge of town planning before any fragmentation [
19,
20]. This should help to monitor compliance with planning and has resulted in a dramatic increase in the area of the city. Thus, since the 1970s, the extension of Greater Lomé towards its peripheral margins has started attracting the attention of researchers, who have not hesitated to develop research themes within the framework of numerous scientific works. The literature indicates that an increase in the population results in accelerated demand for natural resources, resulting in ecosystem and landscape degradation [
21].
Finally, it is important to note that the ecosystem and associated landscapes provide important services, such as oxygen production, carbon sequestration, flood control, food, and cultural services. These landscapes provide urban dwellers with opportunities for tourism and recreation [
22]. This growth is characterized by the extension of the outskirts described as an “unfinished landscape” where facilities are lacking [
23,
24].
4. Discussion
This study revealed that the changes in LULC concern three (03) categories: dense zone (heavily built-up area), moderate-density zone (moderately built-up area), and low-density zone (weakly built-up area). Indeed, as one progresses from the countryside towards the center of a city, one can observe a tightening of the plot design, a convergence and densification of the communication networks, and a change in the assignments of which the most spectacular is undoubtedly the densification of buildings [
45]. Similar results were obtained during the study of landscape dynamics in the upper Ouémé basin (Benin Republic) using Landsat imagery [
46]. Land use and land cover change is one of the major driving forces of global environmental change and is of major concern because of its impacts on various sectors of the economy [
47]. These changes take place temporally and spatially such as the extent of area and the intensity of LULC. It appears that human activities have caused an increase in land utilization, change, and alteration [
7]. Some large areas of Greater Lomé and mainly natural vegetation have been turned into “high-density” and “low-density” areas due to an increase in pressure from building activities. This pressure leads to a considerable loss of biodiversity due to the destruction of many natural habitats. This confirms the idea that the Earth’s surface is affected by the presence of anthropogenic activities in specific areas [
48,
49], exacerbated by the anthropocentric perspective of several societies [
50]. The increase in the built-up area observed during the study period is a result of the construction of some buildings, roads, and infrastructure development as well as the high demand for land for settlements by the growing population in Greater Lomé. The population increase is mainly due to a high influx of people from other parts of the country for jobs and income generation opportunities [
7].
The rapid urbanization of Greater Lomé has also led to a reduction in the proportion of land and degraded vegetation in peripheral areas in favor of buildings. It is recognized that the extension of urban areas can be influenced, among other things, by the configuration of space, in particular their accessibility and availability [
51]. In addition, ref. [
52], through his study on the spatio-temporal analysis of the dynamics of landscape conversion along the urban–rural gradient in Lubumbashi, revealed that the increase in the proportion of buildings to the detriment of vegetation in the landscape of peri-urban areas makes building space more limited in these areas. This could lead to land saturation, probably followed by land conflicts.
In the outlying areas as well as in the city center of Greater Lomé, the extent of the phenomenon of urbanization is considerable. The needs regarding housing and equipment accumulate from year to year. Like the results of this study, those of [
53] on the Mediterranean coast of north-eastern Morocco showed the importance of socio-economic and political factors in the artificialization of the peri-urban spaces. Indeed, urban expansion is causing a decline and relocation of agricultural activities, in particular market gardening and tree farms, a large part of whose production is intended for the market of Greater Lomé. The strong urban expansion is likely to aggravate the problems of mobility, particularly those of the populations of the outlying districts whose individual means of transport are limited. In fact, there is an imbalance in the spatial distribution of infrastructure, equipment, and services between the north, west, and east of downtown Greater Lomé. These environments constitute peripheral zones where the urban extension continues.
The observation of the geomorphological landscape of Togblé, Adétikopé clearly shows that most of the agglomerations established in the alluvial plain regularly suffer from seasonal flooding. The modification of the natural conditions of runoff caused by each human development has consequences on the dynamics of the watercourse. Clearly, the hydrology of the Zio is deeply affected by its decimetric variations, with the consequence of increasing hydrological risks, in particular the frequency of floods. Despite the frequency of these risks, the lack of recent and continuous discharge data is a serious handicap for the detailed analysis of the impacts of LULC changes on flood risk in this area. Also, the current discontinuous series of flows has enabled flood frequency analysis of the Zio River. Gracius [
54], in his study on the analysis of vulnerability to flood risk and land-use planning in the Commune of Cap-Haïtien, showed that this phenomenon of peri-urbanization could lead to an upsurge in flooding.
To these harmful practices, which have repercussions on the ecology of the river, in this case, the morpho-dynamics of the bed, several other human activities are added, which constitute, in reality, factors of aggravation of the floods, particularly the construction of houses and buildings in the bed of the Zio River. The same is true for the surrounding lowlands, which change the initial geomorphological characteristics of the plain. This aspect has been mentioned by other authors, particularly the development of human activities likely to alter the environment, which remains much more evident at a distance closer to urban centers [
51]. This urbanization profoundly modifies the natural conditions of the water flow, which can cause flooding in these environments.
Scientific information on the spatial dynamics of built-up areas integrating the temporal dimension in Greater Lomé is, therefore, of great importance for decision-makers evaluating urban land use and planning decisions and for the scientific community discovering the causes and effects of land use changes on the management of urban spaces in Togo. However, in this study, spatial resolution is a key factor that can affect image quality and mapping accuracy. The mapping of built-up areas from medium spatial resolution Landsat images can, therefore, limit class discrimination and affect classification accuracy. These medium spatial resolution images may not be able to provide accurate information on the density or distribution of buildings in this area, even though the PCA were calculated on images composed of indices derived from the primary image channels, maximized band information, and eliminated noise. Indeed, the quality of the classifications was assessed by calculating the confusion matrix [
36] and the Kappa K index proposed by [
39]. The Kappa index is expressed as the probability of correct classification on a scale of 0 to 1.
The Random Forest (RF) algorithm was used for mapping on the basis of over 300 training pixels, where classes were determined during the field survey. The validation of the classification was based on control points collected in the field. The Random Forest (RF) algorithm, developed by [
30] was chosen for its good land use prediction capabilities [
31] in the case of temporal analysis [
32]. Several authors have shown that land cover classifications using RF outperform classifications using other types of algorithms, such as maximum likelihood classification [
32]. The RF provides an algorithm for estimating missing values and the flexibility to perform several types of data analysis, including regression, classification, survival analysis, and unsupervised learning [
33].
This is a non-parametric supervised classification algorithm that combines the decision tree algorithm with an aggregation technique. It is included in the “Random Forest” package of the “R” software. (version 4.3.1). The algorithm randomly selects a sample of observations and a sample of variables several times to produce a large number of small classification trees. These small trees are then grouped together and a majority voting rule is applied to determine the final category [
30]. In order to maximize the band information and eliminate noise so that the discrimination of the classes studied can be improved, a PCA was calculated on the images composed of indices derived from the primary channels of the satellite images and the main bands used. The indices used included the Normalized Built-up Difference Index (NDBI), the Soil Adjustment Vegetation Index (SAVI), and the Normalized Moisture Difference Index (MNDWI) [
55].
5. Conclusions
This study has highlighted the interest in using Landsat images to study the evolution of human habitats in urban and peri-urban areas in order to improve the understanding of their dynamics over time. A meticulous choice of satellite images and the method of classification enabled the obtainment of a clear and relevant rendering. The results showed that the dynamics of land use along the urban–rural gradient were characterized in 13 years (between 2007 and 2020) by a clear progression of buildings to the detriment of vegetation in the peri-urban zones. They provided a good understanding of the dynamics of these changes and indicated a strong dynamic in the landscape structure of Greater Lomé, marked by a rapid extension of built-up areas. Furthermore, the results of this study showed a marked extension in the peripheral areas of Greater Lomé, particularly towards the north and west, to the detriment of agricultural and wooded areas. In addition, towards the east, an evolution of the buildings was observed, but it was not continuous. The presence of the lower Zio valley, which constituted a green band on the images, caused the discontinuity of the evolution of the buildings in the east of the study area. However, the observation of the satellite images showed that the evolution of the buildings has narrowed this band of discontinuity, which was more stretched in 2020 than in 2016.
Finally, direct observations of the entire study area showed that the minimal conversions observed for built-up areas to other land use classes could be justified, for the most part, in places where houses have been washed away by floods. These places, which have become uninhabitable, were subsequently occupied by vegetation. It becomes necessary to carry out studies on the effects of the occupation of the lower Zio Valley on the dynamics of floods in Greater Lomé in order to better understand the problems and suggest solutions for mitigating their negative consequences, which have become more severe.